I personally found my old job's ESPP a waste of time. Marginal discount (5%), a flat $40 per sale fee, no look back provision (purchase price was always the average price on 2/28, 5/31, 8/31, and 11/30, less the discount), and it took them 3 weeks to issue the stock after the purchase date, so taking a "maxing and selling immediately" approach had some substantial short term price risks.
Plus, it was somewhat risky to put so many eggs in the basket of my employer. After all, they were also the source of my salary, so if this S&P 500 company were to tank and have layoffs, I'd be out my salary and my savings in the ESPP would be worth far less.